1. Field of the Invention
The present invention relates to a semiconductor device using an SOS (Silicon On Sapphire) substrate and a manufacturing method thereof.
2. Description of the Related Art
A conventional semiconductor device is one of such a type that when a semiconductor element having a CMOS (Complementary MOS (Metal Oxide Semiconductor)) structure is formed in a monocrystal silicon layer of an SOS substrate, an insulating film layer constituted of a silicon oxide film is provided between a sapphire substrate and the monocrystal silicon layer, and a P channel MOS element (called “pMOS element”) is formed in monocrystal silicon on the insulating film layer thereby to prevent diffusion of aluminum from the sapphire (Al2O3) substrate to enhance off leak characteristics, whereas an N channel MOS element (called “nMOS element”) is formed in its corresponding monocrystal silicon layer directly formed in the sapphire substrate, and P-type concentration of a channel region of the nMOS element, corresponding to a P type diffusion layer is enhanced by aluminum corresponding to a P type impurity diffused from the sapphire substrate to improve off leak characteristics (refer to, for example, a patent document 1 (Japanese Unexamined Patent Publication No. 2001-267577 (see from the paragraph 0005 in page 2 to the paragraph 0013 in page 3 and FIG. 1)).
However, the above prior art is accompanied by a problem that since the nMOS element is formed in the monocrystal silicon layer directly formed in the sapphire substrate, compressive stress incident to epitaxial growth of silicon on the monocrystal silicon layer and the sapphire substrate occurs in the monocrystal silicon layer and hence the mobility of electrons moved through a channel formed in a channel region formed in the monocrystal silicon layer as the P type diffusion layer is decreased so that an on current is reduced, thereby degrading transistor characteristics.